In most cases, there is a phase angle difference between a sinusoidal current supplied to a power grid by an alternating current generator and a voltage at the generator's terminals. This phase angle difference between the voltage and the current is due to the nature of the load on the power grid. For a purely resistive load (i.e., having no energy storage properties) the voltage and current are in phase, i.e., the current and voltage reverse their polarity simultaneously and a direction of power remains fixed and does not reverse.
For a purely reactive load the current and voltage are 90 degrees out of phase and the net power flow is zero as the power flows to and returns from the load due to the energy storage features of the reactive load. If the load is purely inductive, the current lags the voltage by 90 degrees. A lag angle is between 0 and 90 degrees for a load that is both inductive and resistive. For a purely capacitive load the current leads the voltage by 90 degrees. A lead angle is between 0 degrees and 90 degrees for a load with both resistive and capacitive properties. Thus the magnitude of the phase angle difference depends on the resistance, inductance and capacitance of the load to which the generator supplies power and on the characteristics and operating point of the generator.
For a load with both reactive and resistive properties, the current phase angle (relative to the voltage phase angle) can be resolved into an in-phase component (i.e., in-phase with the voltage) and an out-of-phase component (i.e., a 90 degrees out-of-phase with the voltage). Thus the component of the current that is in phase with the voltage results in the delivery of real or active power into the load. The component of the current that is phase shifted by 90 degrees from the voltage, referred to as reactive power, performs no useful work. The energy associated with this current flows from the generator to the load and then back to the generator, resulting in a net zero energy at any point in the circuit.
The generation and control of reactive power in an electrical transmission system is important to the overall power system efficiency and stability. Capacitors, capacitive loads and capacitive compensators are considered to generate reactive power. Inductors, inductive loads (e.g., transformers and motors) and inductive compensators are considered to consume reactive power. Also, lightly loaded transmission lines generate reactive power and heavily loaded lines consume reactive power.
Electric power transmission systems are designed recognizing that the three power system parameters of impedance, voltage and phase angle between the current and voltage cannot be controlled fast enough to accommodate dynamic system conditions. Furthermore, available control devices usually compensate or control only one of the three variables. Thus transmission systems having been designed with fixed or mechanically-switched series and shunt reactive compensations, together with voltage regulating and phase-shifting transformer tap changers, to optimize line impedance, minimize voltage variation, and control power flow under steady-state or slowly-changing load conditions. The dynamic system problems have been typically addressed by over-design, i.e., designing the system with generous stability margins to recover from worst case contingencies resulting from faults, line and generator outages, and equipment failures. This practice of over design results in the under utilization of the transmission system.
In recent years, energy demands, environmental considerations, right-of-way access, and cost issues have delayed the construction of both generation facilities and new transmission lines. This has necessitated a change in the traditional power system concepts and practices; better utilization of existing power systems has become imperative. But higher utilization of power transmission systems, without an appreciable degradation in the reliability of the supply of electric power, is possible only if the power flow can be controlled rapidly following dynamic system disturbances.
Electric power is provided by a rotating generator driven by a turbine. The mechanical output power of the turbine cannot be changed quickly to balance the mechanical power with a rapidly changing electrical power demand. Consequently, the generators are forced to accelerate or decelerate responsive to changes in power demand. For example, an electrical demand greater than the electrical generation causes the generator to slow down and the frequency of the electrical energy on the distribution system may drop. Conversely, when excess electrical energy is available the generator accelerates and the electrical system frequency increases.
This change in the generator's rotational speed results in a corresponding angular position change (also referred to as a rotor power angle change), with respect to a constant angular position maintained elsewhere on the transmission line by other generators, i.e., typically a large, distant, undisturbed generator also referred to as an “infinite” bus. The angular position change between generators alters the amount of electric power transmitted. Once the disturbance is over (e.g., a fault cleared, new transmission system configuration, new power generation level or new load demand established) the disturbed generators try to reach a new angular position appropriate to the new steady-state condition of the power system. However, the generators together with the associated turbines have significant rotational inertia and, for this reason the new angular position is usually reached only after an “overshoot” or oscillation period. These transient angular changes and oscillations, of course, manifest themselves as transient electric power changes and oscillations. In the extreme case, these transient changes cannot be stabilized; the equilibrium between the available mechanical power and transmitted electric power cannot be reestablished and the angular “overshoot” increases. The generator then accelerates until it is automatically shut down when a maximum rotational speed is reached. The angular oscillation may also continue or even increase due to insufficient oscillation damping in the power system. These undamped oscillations may ultimately cause the power system to be shut down by tripping of protective devices.
One measure of the ability of a power system to provide electric power to meet load demand is power system “stability.” “Transient stability” refers to the capability of the power system to recover normal operation following a major disturbance (fault, loss of generation, etc.). “Dynamic stability” refers to the capability of the power system to recover normal operation following a minor disturbance that initiates power oscillations. Thus a dynamically stable power system has positive damping to damp or remove the power oscillations.
Various devices are in use to stabilize bulk-power transmission and distribution systems and to improve the transient and dynamic stability of the power system. These devices, referred to generally as flexible AC transmission system (FACTS) devices can provide rapid voltage regulation and power flow control. FACTS devices include: static-var compensators (SVC), static synchronous compensators (STATCOMS), and thyristor-controlled series capacitors (TSCSs). Use of these devices to limit effects of power system impedance changes permits loading of the transmission facilities to levels approaching their ultimate thermal capacity. These devices may regulate system voltage and/or provide power modulation to damp electromechanical oscillations. In any case, the FACTS devices control the voltage, impedance or phase angle on the power system.
STATCOM devices lack any substantial real energy storage and are simply voltage-sourced inverters that regulate voltage of the grid via a step up transformer. In present devices, only transient energy storage is provided by a relatively small DC capacitor that is used to exchange reactive power between phase conductors of the power system. Since the STATCOM can only regulate voltage, the STATCOM is limited in the degrees of freedom and sustained power damping oscillation actions that it can contribute to the grid.
For example, one STATCOM was developed specifically for power oscillation damping for inter-area power oscillations by modulating the voltage at the interconnection. It is known that inter-area power oscillations occur on transmission systems with long lines and large physical distances between major generation sources. Typically, after a disturbance, groups of generators in a first geographic region swing against another group of generators in a second geographic region separated from the first region by a series of long transmission lines. Normally, these oscillations are of a very low frequency (typically between 0.1 and 0.7 Hz) and are poorly damped in the absence of supplemental damping.
Inter-area power oscillations are a particularly common phenomenon in the US western states, Canada, and other regions with low power generation density and long transmission lines between generating units. To damp these inter-area oscillations, synchronous generators in these regions are typically required to have power system stabilizers (PSSs) to provide supplemental damping to ensure the oscillations are damped before they cause system instabilities.
Wind turbines exploit wind energy by converting the wind energy to electricity for distribution to users. Several factors must be considered in identifying a wind turbine site and designing the turbine, including, tower height, control systems, number of blades, and blade shape. Wind turbines are typically sited at isolated locations where the grid may be regarded as relatively “weak” due to the few generating plants and long distances between plants. A “weak” system may be characterized by a relatively low short circuit strength, e.g., less than about 10 kA, and/or a variations in voltage at different points on the system.
The rotor of a fixed-speed wind turbine is turned by the wind-driven blades and operates through a gear box (i.e., a transmission) at a fixed rotational speed. The fixed-speed wind turbine is typically connected to the grid through an induction (asynchronous) generator that generates real power. The rotor and its conductors rotate faster than the rotating flux applied to the stator from the grid (i.e., higher than the synchronous speed). At this higher speed, the direction of the rotor current is reversed, reversing the counter EMF generated in the stator windings, and by generator action causing current (and real power) to flow from the stator windings. The frequency of the generated stator voltage will be the same as the frequency of the applied stator voltage providing the excitation. The induction generator may also use a capacitor bank for reducing reactive power consumption from the power system.
The fixed-speed wind turbine is simple, reliable, low cost and well-proven. But its disadvantages include uncontrollable reactive power consumption (as required to generate the stator rotating flux), mechanical stresses, limited power quality control and relatively inefficient operation. In fact, wind speed fluctuations result in mechanical torque fluctuations that then result in fluctuations in the electrical power on the grid.
In contrast, the rotational speed of a variable speed wind turbine can be continuously adapted to the wind speed, with the blade speed maintained at a relatively constant value corresponding to a maximum electrical power output through the use of a gear box disposed between the wind turbine rotor and the generator rotor. The variable speed wind turbine is typically equipped with a synchronous generator (the output of which is a variable frequency AC) and connected to the grid through a power converter system that rectifies the incoming variable AC to DC and inverts the DC to fixed frequency 60 Hz AC. Variable speed wind turbines have become widespread due to their increased efficiency over fixed speed wind turbines.
The present invention relates to a transmission system power flow controller that employs wind turbine-generated electricity to control and stabilize power flow on a transmission line.